The present invention claims priority from Japanese Patent Application No. 10-063170 filed Mar. 13, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an adaptive equalizer, and more particularly to an adaptive equalizer for equalizing a received signal in order to reduce interference between codes.
2. Description of Related Art
Up to now, in a digital communication system such as a mobile radio communication system and the like, a technique of adaptively equalizing a received signal with an adaptive equalizer in order to compensate for degradation in performance of a transmission channel due to interference between codes has been known (Japanese Patent Application Laid-open No. 5-3437, No. 5-316083, No. 5-110617, and the like).
FIG. 9 shows a block diagram of an example of a portable telephone provided with an adaptive equalizer. In FIG. 9, a received signal received by a radio receiving portion 41 is converted into a digital signal by an analog-to-digital (AD) converter 42 and then is supplied to an adaptive equalizer 43 and is applied here with an adaptive equalizing process to be an equalized data signal. An equalized data signal outputted from the adaptive equalizer 43 is supplied to an error correction decoder 44, and is here error-corrected and decoded and then taken out.
FIG. 10 shows a block diagram of an example of a conventional adaptive equalizer used as the above-mentioned adaptive equalizer 43. In FIG. 10, a received signal is inputted into a channel estimating portion 1, an adaptive algorithm operating portion 10, and a Viterbi decoder 3. The channel estimating portion 1 estimates a channel state from an inputted reception signal to find an impulse response of the channel. The adaptive algorithm operating portion 10 updates an impulse response with an adaptive algorithm on the basis of the received signal and the impulse response of the channel estimated by the channel estimating portion 1.
The adaptive algorithm is an algorithm for updating every moment the impulse response of a channel in order to bring the impulse response as close as possible to an actual channel state. What the update is performed on the basis of is entrusted to a reference literature describing in detail adaptive algorithms (xe2x80x9cWaveform Equalizing Techniques for Mobile Digital Communicationxe2x80x9d, pp. 33 issued by TORIKKEPS), but this is simply described as follows.
A result obtained by multiplying an impulse response used in an adaptive equalizing process by a result of the adaptive equalizing process is compared with the received signal. If the impulse response has been correctly estimated, both of them must be equal to each other. A difference between both of them is detected as an error, and this is fed back for updating the impulse response. It is assumed that a parameter to control the update quantity of an impulse response to be updated is referred to as an update quantity control parameter xcexc. An error which is equal in quantity to but larger in xcexc than another error has a larger update quantity in impulse response, while an error which is equal in quantity to but smaller in xcexc than another error has a smaller update quantity in impulse response.
The Viterbi decoder 3 equalizes a received signal by performing a Viterbi decoding process based on the received signal and the impulse response of a channel updated by the adaptive algorithm operating portion 10. The equalized data signal obtained by this Viterbi decoder 3 is outputted to the latter stage and simultaneously is fed back to the adaptive algorithm operating portion 10. Thus, the received signal is taken out as an equalized data signal applied with an adaptive equalizing process.
However, since the above-mentioned conventional adaptive equalizer handles as a fixed value an update quantity control parameter xcexc for controlling the update quantity of an impulse response to be used in updating a result of estimating a channel state according to an adaptive algorithm, the update quantity control parameter xcexc is not optimized adaptively to variation of the channel state, and this causes deterioration in BER (bit error rate) performance.
The present invention has been performed in consideration of the above-mentioned point, and has an object of providing an adaptive equalizer capable of reducing deterioration in BER performance by optimizing an update quantity control parameter xcexc for controlling the update quantity of an impulse response adaptively to variation of a channel state.
In order to attain the above-mentioned object, the present invention provides an adaptive equalizer comprising; a channel estimating means for having a received signal inputted and estimating the impulse response of a channel of the received signal, an adaptive algorithm operating means for adaptively updating the impulse response of the channel estimated by the channel estimating means according to an update quantity control parameter on the basis of an adaptive algorithm, a decoding means for performing a Viterbi decoding process based on the received signal and the impulse response obtained the adaptive algorithm means and outputting an equalized data signal of the received signal, a bit error rate estimating means for estimating a bit error rate of the equalized data signal taken out from the decoding means, and an update quantity control parameter adjusting means for adjusting and supplying an update quantity control parameter on the basis of an estimated bit error rate value calculated by the bit error rate estimating means to the adaptive algorithm operating means.
Since the present invention adjusts and supplies an update quantity control parameter on the basis of an estimated error rate value to an adaptive algorithm operating means, it can estimate a channel state on the basis of the estimated bit error rate value and optimize the value of an update quantity control parameter xcexc adaptively to variation of the channel state.
The present invention provides an adaptive equalizer as described above, further comprising an average power calculating means for calculating the average power of received signals, in which the update quantity control parameter adjusting means determines what class an average power calculated by the average power calculating means belongs to among plural predetermined classes and compares in magnitude a predetermined threshold value corresponding to a class determined by it and the estimated bit error rate value with each other, and updates an update quantity control parameter so that it increases or decreases according to a result of comparison obtained at this time.
Since the present invention determines what class the average power of a received signal belongs to among plural classes and compares in magnitude a predetermined threshold value corresponding to a class obtained by the determination with an estimated bit error rate value, and increases or decreases an update quantity control parameter according to a result of comparison obtained at this time, it can adjust an update quantity control parameter corresponding to the magnitude of an expected bit error rate varying with the strength of a reception electric field even in the same channel state.
Hereupon, the update quantity control parameter adjusting means is characterized by updating an update quantity control parameter so that it increases by one step width which has been initially set when an estimated bit error rate value calculated by the bit error rate estimating means is larger than a threshold value and updating an update quantity control parameter so that it decreases by one step width initially set when an estimated bit error rate value is equal to or smaller than a threshold value.
The bit error rate estimating means according to the present invention is characterized by taking out data corresponding to a training sequence out of equalized data taken out from the decoding means, comparing the data with a predetermined reference sequence, counting the number of bits in those data being different in value from each other, and determining a value obtained by dividing that number of different bits by the number of bits of the data corresponding to the training sequence as an estimated bit error rate value.
The update quantity control parameter adjusting means may limit the upper limit of a value obtained by updating an update quantity control parameter so that it increases to a predetermined maximum value, and may limit the lower limit of a value obtained by updating an update quantity control parameter so that it decreases to a predetermined minimum value.